public static void main(String[] args){
{
Window win= new Window(); // what the heck is a Window object and why are you using it?
win.setSize(800,600);
}
public void paint(Graphics g){
{
screen.drawLine(300,250,725,650); // why are you drawing with "screen"???
screen.drawLine(400,400,400,800);
}
}

Sorry to be brutally blunt, but this code looks like it was put together by blind monkeys. You can't just randomly type in code and expect it to work mathemagically. Instead, you should plan your code before you type it.

If you want to see code that actually does screen graphics, you'll find lots at the Sun Swing tutorials. That's likely your best bet since it will contain both working code samples and tutorials so you can understand what your coding and adapt it to your desires (and be able to fix errors that come up too). Much luck.